You can set your conditional forwards to a number that won't answer. You used to be able to get Tech Support to disable DCF, which allowed the customer to turn voicemail off and on but I've been told that they won't do that any more.. So, you're stuck in the-customer-is-an-idiot mode.

In the case of Google voice you are not deactivating voicemail but forwarding to another number other than the T-Mobile voicemail system. In that case you just change the conditional call forwarding to the service you want to use instead.

I was unaware that they didn't do that anymore. It has been a while since I have heard of anyone requesting it.

Step 1: Go to your home screen and tap on the keypad icon at the bottom.

Step 2: Once the keypad is launched, type ##002# and then click on the Call button. There is no cause for worry since the iPhone will not actually make the call, but will display the settings of your voicemail.

Step 3: You will be presented with three different messages* Voice Call Forwarding* Data Call Forwarding* Fax Call forwarding.

Do not do anything, but simply tap on the “Dismiss” tab at the bottom of the screen. You will have successfully disabled your voicemail for all types of calls.

I disabled my voicemail because I got tired of bill collectors, telemarketers, and robots leaving messages or at least staying on the line long enough to register a blank message. Blocking their numbers doesn't help when they can still leave a message. If someone is calling and they aren't a pest, I'll see their number for me to call back (especially if they are a contact) or they can text me or email me. I'd probably turn it back on if T-Mobile gave the option to not allow blocked callers to leave messages.